Abstract
The sound attenuation characteristics of the box type sound attenuators for air outlet units which are used in the high velocity air duct systems, were investigated theoretically and experimentally. The box type sound attenuators are considered as the acoustic filters with expansion-chambers and bends. In this paper, the effects of sectional shape of the boxes and of outlet size and position on attenuation are discussed. 1. In the case of the large aspect ratio of the section of the box, a practical limitation of the application of the theory was investigated. When the long side of section of the box was greater than the length of the sound waves, the theoretical values of attenuation did not agree with the observed magnitude. The inlet and outlet were located at the centers of the opposit walls of the box. 2. The equation is presented for the attenuation characteristics of the expansion-chamber attenuators with different inlet and outlet areas. The measurements were made for the boxes with various sizes of outlets and a constant inlet area. The theoretical attenuation showed fair agreement with that of observed. 3. When the inlet and outlet are located at adjacent walls of the box respectively, the position of outlet affect the attenuation characteristics of the box type attenuator. If the outlet locates at the far end from the inlet, the attenuation characteristics will be the same as that of the box having inlet and outlet in series at the opposit walls, and if the outlet locates nearest to the inlet, the attenuation by resonance will take place. When the outlet located between those distances, the attenuation characteristics will be represented by combination of both of the expansion-chamber and of the resonator. Best attenuation was obtained, when the outlet located at the center of side wall.
